Reversible switching between superhydrophobic states on a hierarchically structured surface

Verho, Tuukka; Korhonen, Juha; Sainiemi, Lauri; Jokinen, Ville; Bower, Chris; Franze, Kristian; Franssila, Sami; Andrew, Piers; Ikkala, Olli; Ras, Robin H. A.

doi:10.1073/pnas.1204328109

Cited by 265 publications

(273 citation statements)

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“…In future, we foresee that the magnetically controlled normal force enables investigation of transitions from the metastable Cassie-Baxter state to the stable high-friction Wenzel state 12,41 . The oscillatory method may also serve as an efficient tool for studying the microscopic origins of the wetting dynamics, including the role of the contact line pinning and unpinning that leads to the 'stick and slip' behaviour and CAH 42,43 .…”

Section: Discussionmentioning

confidence: 99%

“…Artificial superhydrophobic coatings have been developed and used in self-cleaning 5,6 , antifogging 7 and anti-icing 8 materials, non-wetting textiles 9 , heat-transfer interfaces 10 , drag-reducing coatings 11 , and for storing and displaying information 12 . Generally, superhydrophobicity arises from a combination of surface roughness and a low-energy coating 6 .…”

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confidence: 99%

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Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

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The recently demonstrated extremely water-repellent surfaces with contact angles close to 180°with nearly zero hysteresis approach the fundamental limit of non-wetting. The measurement of the small but non-zero energy dissipation of a droplet moving on such a surface is not feasible with the contemporary methods, although it would be needed for optimized technological applications related to dirt repellency, microfluidics and functional surfaces. Here we show that magnetically controlled freely decaying and resonant oscillations of water droplets doped with superparamagnetic nanoparticles allow quantification of the energy dissipation as a function of normal force. Two dissipative forces are identified at a precision of B10 nN, one related to contact angle hysteresis near the three-phase contact line and the other to viscous dissipation near the droplet-solid interface. The method is adaptable to common optical goniometers and facilitates systematic and quantitative investigations of dynamical superhydrophobicity, defects and inhomogeneities on extremely superhydrophobic surfaces.

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Section: Discussionmentioning

confidence: 99%

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confidence: 99%

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

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“…Hence, a key to optimize water-repellent materials and guarantee their promising technological applicability would be to suppress the energy barrier between Wenzel and Cassie states. In this context, it is worth exhibiting monostable Cassie states where even an accidental transition to the undesired Wenzel state, due to force fluctuations such as encountered in an impact (37) or to pressure applied on the liquid (16,17,33,(46)(47)(48)(49)(50), can be repaired owing to the absence of barrier between both states. Because reported Wenzel-to-Cassie (W2C) transitions generally involve either an external energy input (17,49,(51)(52)(53) or a potential energy release (35,(54)(55)(56), monostable Cassie states might be seen as unreal (57), but we describe here such situations and criteria for achieving them.…”

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confidence: 99%

Monostable superrepellent materials

Quéré

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

104

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Superrepellency is an extreme situation where liquids stay at the tops of rough surfaces, in the so-called Cassie state. Owing to the dramatic reduction of solid/liquid contact, such states lead to many applications, such as antifouling, droplet manipulation, hydrodynamic slip, and self-cleaning. However, superrepellency is often destroyed by impalement transitions triggered by environmental disturbances whereas inverse transitions are not observed without energy input. Here we show through controlled experiments the existence of a "monostable" region in the phase space of surface chemistry and roughness, where transitions from Cassie to (impaled) Wenzel states become spontaneously reversible. We establish the condition for observing monostability, which might guide further design and engineering of robust superrepellent materials.repellency | Cassie state | monostability | interfaces | dewetting W ater repellency describes the ability of materials to repel water and make it flow with negligible friction and adhesion, compared with usual situations. It is achieved by combining chemical hydrophobicity with micro-and/or nanotextures (1, 2). Water meeting such materials remains at the textures' tops, which generates a composite interface made of hydrophobic solid and air trapped inside the textures (Cassie state) (3). As a consequence, water hardly contacts these solids on which its dynamical behaviors are spectacular (4-7). Repellency also holds if the liquid has a higher surface tension than water (salty water, mercury); using special texture designs, it can even be extended to liquids with smaller surface tension (8), and/or to water at small scale (such as dew) (9, 10), so it was proposed (11) to call such materials "superhygrophobic," from the Greek "hygros" meaning humid.Repellent materials are classically found in nature, in particular at the surface of many plants and insects, two situations where the control of water is crucial for surviving (1, 12). It was reported that these natural surfaces often (yet not always) exhibit dual structures, with microbumps on the scale of 10-100 μm coated by nanostructures of typically 100 nm (1). This results in an amplification of static (13-17) and dynamical (18) repellency, because we can then expect the generation of composite solid/air interfaces at different scales--where we mean by amplification an improved nonwettability (13-15, 17) and a smaller adhesion (16,18,19), two factors that contribute to the liquid mobility.This field of research has been very active for about 20 years, with theoretical, experimental, and computational viewpoints. Researchers discussed the surface energy of materials having different kinds of structures (20-23), various adhesion properties (24-26), diverse geometries of solid-liquid-vapor contact lines (27, 28), or flow interactions between the liquid and its substrate (29-32). In many studies, model hydrophobic textures (such as lines or pillars) were considered, and wetting was found to be usually "bistable": Depending on the history o...

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“…We introduced a dual-level topography by coating a standard, micropillared substrate with nanofilaments (Fig. 2a, right) [6]. The pressure-induced collapse of the standard "micro-Cassie" state now resulted in a Wenzel-like state that has water between the micropillars, and we call it "nano-Cassie" state because there remains a nanoscopic air layer present keeping the wetted solid fraction small.…”

Section: Slippery and Never Wet Features 32mentioning

confidence: 99%

Slippery and never wet

2017

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Superhydrophobic surfaces let water droplets roll off with low friction and falling droplets rebound, leaving the surfaces completely dry. Such extremely water repellent surfaces are found in nature on lotus leaves, the legs of water striders and feather coatings of birds, and portray a beautiful example of ingenious biological design. They provide an exciting research avenue for physicists and materials scientists aspiring to understand and mimic nature.. Photo by Mika Latikka A physicist's approach to superhydrophobicitySuperhydrophobicity, that is the extreme fear of water, is a fascinating surface property found in nature on many plants and insects. The strong water repellency is often crucial for surviving in harsh conditions: the superhydrophobic surface on submerged insects, for example, traps small air pockets on their bodies that act as an external lung to allow gas exchange and breathing underwater. Barthlott and Neinhuis realized in 1997 that the intriguing self-cleaning nature of a lotus leaf arises from the combination of surface microroughness and a low-surface-energy wax nanocrystal coating (Fig. 1a) [1]. This discovery opened an exciting avenue for scientists aspiring to mimic the designs found in nature to manufacture artificial superhydrophobic substrates.

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Reversible switching between superhydrophobic states on a hierarchically structured surface

Cited by 265 publications

References 26 publications

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

Free-decay and resonant methods for investigating the fundamental limit of superhydrophobicity

Monostable superrepellent materials

Slippery and never wet

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